[There is a] long conceptual chain that starts from simple, ostensive definitions and rises to higher and still higher concepts, forming a hierarchical structure of knowledge so complex that no electronic computer could approach it. It is by means of such chains that man has to acquire and retain his knowledge of reality. [RM, 18]
To know the exact meaning of the concepts one is using, one must know their correct definitions, one must be able to retrace the specific (logical, not chronological) steps by which they were formed, and one must be able to demonstrate their connection to their base in perceptual reality. [IOTE, 50]
That some concepts are "wider" than others — that animal, for example, is wider than mammal, and mammal wider than deer — is something so trivial that hardly anyone has bothered making a fuss about it before Rand. But the way some Objectivists talk about the hierarchy of knowledge, you would think that only Rand noticed it, while everyone else is in denial that concepts have any such structure. "Knowledge is hierarchal," Rand's disciples keep insisting; to which the obvious retort is, "So what!" The problem with Rand's hierarchy of knowledge is not that it is wrong but that Objectivists exaggerate its importance.